Interactions between charged particles in solution play a fundamental role in many branches of science and technology, including colloidal science, polymer science, membrane technology, molecular biology, biotechnology etc. However, despite their fundamental and ubiquitous nature some problems in this field have defied solution for decades. One of these long standing puzzles is the attractive interaction between similar particles in solution in the presence of multivalent ions, also known as “like-charge attraction”. Examples include DNA condensation by Z=3 and 4 cations and aggregation of charged fibers, membranes, proteins, polymers, and colloids.

We have measured, for the first time, the attractive interaction as a function of separation between two similar particles in the presence of multivalent cations.We have found that for pure NaCl the interaction was always repulsive. Upon addition of cobalt hexamine ions, the repulsion was gradually suppressed and a pronounced attraction developed at short distances. Higher concentrations of cobalt hexamine turned the attraction back into repulsion. The analysis revealed an exponential dependence of the force upon separation with characteristic lengths considerably shorter than the corresponding calculated Debye-Hückel lengths. Several previously proposed models for like-charge attraction were compared with the experimental data, but none could account for the measured force curves. As such, the measured force curves facilitated indispensable testing of models for the widely observed attraction between similarly charged objects in the presence of multivalent ions.